400 Prof. J. H.,PoyntiDg-on 



momentum of the incident beam. Resolve AB into AE 

 parallel and EB normal to the surface. Then, since the 

 momentum AB disappears as light-momentum, there must 



Fig. 3. 



Fig. 4. 



Fig. 5. 



be a normal force EB on the surface and a tangential force 

 A_h parallel to the surface. I have lately* described an 



for P ce r AE nt ^^ ^^ ^ GxistenCe o£ the tangential 

 * Phil. Mag. Jan. 1905, p. 169. 



